Custom Effect

A method to write custom GLSL pixel shader based effect strips would be great.

Complexity can quickly explode if we're not careful, but it should be relatively simple to pass some basic data from a hidden vertex shader (or maybe we can modify that too, not sure to what end though) and some uniforms: screen pixel, screen uv, source strip texture, strip start/end/current time.

We could also make a special case for custom transition strips where we additionally pass the target strip texture.

Eventually we can allow shader/effect writers a way to add their own uniforms the end user could modify in the strips UI.

This would be a great addition even if we get a Send to Compositor effect strip (yes please!)